For example, one of the prior art magnetic head driving circuits, wherein the magnetic field modulation system is adopted, is found in Japanese Laid-Open Patent Application No. 63-94406. Here, the following description will discuss this prior art system in detail.
When a 2-7RLL code in FIG. 10(a) is recorded using the NRZI recording system, appearance of bit "1" makes a recording current of a magnetic head reversed. In a magnetic head driving circuit shown in FIG. 9, the reversing of the recording current is conducted by switches 6 and 7.
More specifically, when the switch 6 is turned on (see FIG. 10(b)) with the switch 7 off (see FIG. 10(c)), a current I.sub.1 is connected to ground from a dc power source 3 by way of an auxiliary coil 2 and the switch 6. Also a current I.sub.2 is connected to ground from a dc power source 5 by way of an auxiliary coil 4, a magnetic head coil 1 and the switch 6. Simultaneously as the switches 6 and 7 are reversed, an induced high voltage appears at a connecting point Q between the auxiliary coil 4 and the switch 7. As a result, a driving current I.sub.x, different from the current I.sub.2, flows through the magnetic head coil 1 in a direction from Q to P.
On the other hand, when the switch 7 is turned on with the switch 6 turned off (see FIG. 10(b) and (c)), the current I.sub.2 of the auxiliary coil 4 is connected to ground from the dc power source 5 by way of the auxiliary coil 4 and the switch 7. Also the current I.sub.1 of the auxiliary coil 2 is connected to ground from the dc power source 3 by way of the auxiliary coil 2, the magnetic head coil 1 and the switch 7. Simultaneously as the switches 6 and 7 are reversed, a high induced voltage appears at a connecting point P between the auxiliary coil 2 and the switch 6. As a result, a driving current I.sub.x, different from the current I.sub.1, flows through the magnetic head coil 1 in a direction from P to Q.
Here, FIG. 10(h) and (g) respectively indicate loads in response to "on" and "off" of the switches 6 and 7 when the coil 1 is seen from the side of the auxiliary coils 2 and 4. As illustrated in these drawings, the loads vary to be equal to the value of the impedance (Zx) of the coil 1 or to be zero. However, the inductance (Lx) of the coil 1 is set to be substantially smaller than the inductance (Ld) of the auxiliary coils 2 and 4. That is, the setting is made to satisfy: Lx&lt;&lt;Ld. Therefore, Lx.congruent.0 and Zx.congruent.0. That is, the loads when seen from the side of the auxiliary coils 2 and 4 become virtually zero; thus, the variation of the loads can be virtually ignored.
By the use of the above-mentioned magnetic head driving circuit, information is recorded, for example, by forming recording marks on a magneto-optical disk. The recorded information is then reproduced by an information reproducing circuit in response to a readout signal derived from these recording marks. An example of such an information reproducing circuit is found in Japanese Laid-Open Patent Application No. 1-13658.
This information reproducing circuit is provided with a positive peak-hold circuit and a negative peak-hold circuit. A slice signal is formed by the addition of outputs from both of the peak-hold circuits in an appropriate ratio (for example, 1:1). The level of the readout signal is compared with that of the slice signal; thus, a binary signal is extracted from the readout signal.
However, in the above-mentioned conventional magnetic head driving circuit, when there is a difference in size between the positive component and the negative component of the driving current I.sub.x of the coil 1 (that is, arising a lack of balance), the distance between zero-crossing points in the driving current I.sub.x deviates and the magnitudes of generated magnetic fields in the positive direction and the negative direction become different from each other. As a result, jitter of the recording marks increases.
This problem becomes more obvious in the case where the NRZI recording system for 2-7 RLL code is adopted as a modulation system of the magneto-optical recording-reproduction apparatus. This system, although having an advantage that the recording density can be increased, has a disadvantage that a lot of dc component is contained in the recording current. Therefore, if a recording signal obtained by this system is inputted to the conventional magnetic head driving circuit, the above problem is inevitably presented.
Moreover, when it is arranged to increase the frequency of the recording signal in order to enhance the data transfer rate, Ld should be minimized. As Ld is reduced, Ld becomes as small as Lx. This results in variation of the loads.
Consequently, as illustrated in FIGS. 10(d) and 10(e), the balance of the currents I.sub.1 and I.sub.2 is upset, thereby resulting in a difference therebetween. Since the distance between zero-crossing points in the driving current I.sub.x deviates and the magnitudes of the driving current I.sub.x in the positive direction and the negative direction become different from each other, jitter of the recording marks increases, thereby reducing the reliability of reproduced data.
More specifically, in the case where the positive component and the negative component of the driving current I.sub.x of the coil 1 are different from each other in size, if recording marks (indicated by a solid line in FIG. 11(c)), which have been recorded according to 2-7 RLL code (see FIG. 11(a)), are read out by an optical head (not shown), a readout signal, indicated by a solid line in FIG. 11(d), will be obtained.
On the other hand, in the case where the positive component and the negative component of the driving current I.sub.x of the coil 1 are the same in size (that is, balance each other), if recording marks (indicated by a dashed line in FIG. 11(c)) are read out by the optical head (not shown), a readout signal, indicated by a dashed line in FIG. 11(d), will be obtained.
In the conventional information reproducing circuit, a slice signal is obtained by the use of peaks in the positive direction and in the negative direction of an input signal. For example, when the slice signal is formed by using an intermediate level between the peak in the positive direction and the peak in the negative direction (see FIG. 11(d)), an output signal (see FIG. 11(e)) of a comparator (not shown) contains jitter to an extreme extent. In comparison with the case where the positive component and the negative component of the driving current I.sub.x balance each other, a period of "1" of the output signal of the comparator is shortened while a period of "0" thereof is lengthened. This forms one of the reasons that reproduced data becomes deteriorated.
Additionally, the above example has described the case where recording marks are shorter than non-recording marks; however, on the contrary, another case may be considered, wherein recording marks are longer than non-recording marks. Here, in comparison with the case where the positive component and the negative component of the driving current I.sub.x balance each other, the period of "1" of the output signal of the comparator is lengthened while the period of "0" thereof is shortened.
Furthermore, the above example has described the case where the slice signal is obtained from the intermediate level between the peak in the positive direction and the peak in the negative direction. However, in another case, if a slice signal, which is obtained by addition of the peaks in the positive direction and in the negative direction in an appropriate ratio, is used, in other words, if a slice signal having a level that deviates from the intermediate level is used, jitter will be reduced in either "0" or "1"; however, jitter will be increased in the other "1" or "0".